power quality

Universidade de Coimbra
Power Quality Problems and
New Solutions
Aníbal T. de Almeida
International Conference on Renewable Power and Power Quality ’03
Vigo, April 9 – 11, 2003
University of Coimbra
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Power system constraints
• Random external factors
- Weather
- Human activities
- Animals
-Vegetation
• Increasing share of non-linear loads
• Increasing demand for high PQ
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Main PQ problems
• Voltage sags
• Micro-interruptions
• Long interruptions
• Voltage spikes
• Voltage swells
• Voltage fluctuations
• Voltage unbalance
• Noise
• Harmonic distortion
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Voltage Sags
A decrease of the normal voltage level between 10 and 90% of the nominal
rms voltage at the power frequency, for durations of 0,5 cycle to 1 minute.
Causes:
• Faults on the transmission or distribution
network.
• Faults in consumer’s installation.
• Connection of heavy loads and start-up of
large motors.
Consequences:
• Malfunction of microprocessor-based
control systems (PCs, PLCs, ASDs, etc)
that may lead to a process stoppage.
• Tripping of contactors and
electromechanical relays.
•Disconnection and loss of efficiency in
electric rotating machines.
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Micro-Interruptions
Total interruption of electrical supply for duration from few milliseconds
to one or two seconds.
Causes:
• Opening and automatic reclosure of protection devices.
• Insulation failure, lightning and insulator flashover.
Consequences:
•Tripping of protection devices.
• Loss of information and malfunction of data processing equipment.
• Stoppage of sensitive equipment (such as ASDs, PCs, PLCs).
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Long Interruptions
Total interruption of electrical supply for duration greater than 1 to 2
seconds.
Causes:
• Equipment failure in the power system network.
• Storms and objects (trees, cars, etc) striking lines or poles, fire.
• Human error, bad coordination or failure of protection devices.
Consequences:
• Stoppage of all equipment.
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Voltage Spikes
Very fast variation of the voltage value for durations from a several
microseconds to few milliseconds.
Causes:
• Lightning.
• Switching of lines or power
factor correction capacitors.
• Disconnection of heavy loads.
Consequences:
• Destruction of components
and of insulation materials.
• Data processing errors or data
loss.
• Electromagnetic interference.
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Voltage Swells
Momentary increase of the voltage, at the power frequency, outside the normal
tolerances, with duration of more than one cycle and typically less than a few
seconds.
Causes:
• Start/stop of heavy loads.
• Poorly dimensioned power sources.
• Poorly regulated transformers.
Consequences:
• Flickering of lighting and screens.
• Damage or stoppage or damage of
sensitive equipment.
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Voltage fluctuation
Oscillation of voltage value, amplitude modulated by a signal with low
frequency.
Causes:
•Arc furnaces.
• Frequent start/stop of electric
motors (for instance elevators).
• Oscillating loads.
Consequences:
• Most consequences are common
to undervoltages.
• Flickering
screens.
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of
lighting
and
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Voltage Unbalance
A voltage variation in a three-phase system in which the three voltage magnitudes
or the phase-angle differences between them are not equal.
Causes:
•Large
single-phase
loads
(induction furnaces, traction loads).
• Incorrect distribution of loads by
the three phases of the system.
Consequences:
• The most affected loads are threephase induction machines.
• Increase in the losses.
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Noise
Superimposing of high frequency signals on the waveform of the power-system
frequency.
Causes:
• Electromagnetic interferences.
• Improper grounding may also be
a cause.
Consequences:
• Disturbances on sensitive
electronic equipment.
• May cause data loss and data
processing errors.
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Harmonic Distortion
Voltage or current waveforms assume non-sinusoidal shape.
The waveform corresponds to the sum of different sine-waves with different
magnitude and phase, having frequencies that are multiples of power-system
frequency.
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Harmonic Distortion
Sources of harmonic distortion (1)
Switched mode power supply
Fluorescent lighting
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Harmonic Distortion
Sources of harmonic distortion (2)
3-phase rectifier
Adjustable speed drive
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Harmonic Distortion
Consequences of harmonic distortion (1)
• Conductor overheating (skin effect end proximity effect)
• Neutral overloading (triplen harmonics)
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Harmonic Distortion
Consequences of harmonic distortion (2)
• Increased probability of occurrence of resonance.
• Nuisance tripping of thermal protections.
• Electromagnetic interference.
• Increase in the losses.
• Loss of efficiency in electric machines (e.g. 5th harmonic).
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Harmonic distortion
Consequences of harmonic distortion (summary)
Conductor overheating
Capacitor overheating
or destruction
Transformer
overheating
ASD
Motor
Nuisance tripping of
protections
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Neutral overloading
Motor overheating
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Power Quality characterization
Typical site in USA
In both cases, about 90 %
of
Industrial facilities in Portugal
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PQ
events
have
duration below 1 second.
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Power Quality characterization
92% of PQ disturbances
were voltage sags with
amplitude drops up to 50%
and
duration
below
2 seconds.
Source: EPRI
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Power Quality costs
Direct costs
• Damage in the equipment
• Loss of production and raw material
• Salary costs during non-productive period
• Restarting costs
Indirect costs
• Inability to accomplish deadlines
• Loss of future orders
Non-material inconvenience
• Inconveniences that cannot be expressed in money, such as not
listening to the radio or watch TV
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Power Quality costs
• Business Week (1991) - 26,000 million USD per year in the United States
• EPRI (1994) - 400,000 million USD per year in the United States.
• US Department of Energy (1995) - 150,000 million USD per year for
United States.
• Fortune Magazine (1998) - Around 10,000 million USD per year in United
States.
• E Source (2001) - 60,000 to 80,000 USD per installation, per year for
continuous process industries, financial services and food processing in the
United States.
• European Copper Institute (2001) - 10,000 million EUR per year, in EU in
industry and commerce.
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Power Quality Costs
Cost of momentary interruption (1 minute), in $/kW demand
)
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Power Quality costs
Costs of interruptions vs. duration
Source: Electrotek Concepts
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Power Quality solutions
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Costs of improving PQ
The lowest cost is to implement item 1 at the design stage.
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Grid Adequacy
Many PQ problems have origin in T&D network.
A proper planned and maintained grid will avoid many PQ problems.
- High level of redundancy;
- Cleaning of insulators;
- Trimming of trees nearby power lines…
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Codes and Standards
Need to regulate:
• the minimum PQ level that utilities have to provide to consumers, and
• the immunity level that equipment should have.
Most relevant standards:
• CBEMA curve
• ITIC curve
• IEC 61000
• EN 50160:2001
• IEEE standards
- 519-1992 → Harmonics
- 1100-1992 → Powering and grounding sensitive equipment
- 1159-1992 →Monitoring power quality
- 1250-1995 → Service of sensitive equipment
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CBEMA curve
CBEMA – Computer and Business Equipment Manufacturers Association (1978).
Specifies the maximum and minimum limits that sensitive electronic equipment
should be able to withstand.
Equipment malfunction
Damage on the equipment
Equipment malfunction
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ITIC curve
ITIC – Information Technology Industry Council (1996, revised 2000).
Specifies the maximum and minimum limits that sensitive electronic equipment
should be able to withstand.
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EN 50160
European Norm created by CENELEC (European Committee for Electrotechnical
Standardization).
Defines the voltage characteristics of electricity supplied by public distribution
systems.
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Restoring technologies
Restoring technologies are used to provide the electric loads with ridethrough capability in poor PQ environment.
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Distributed Resources
Distributed Generation (DG)
Energy Storage (restoring technologies)
• Reciprocating engines
• Electrochemical batteries
• Microturbines
• Flywheels
• Fuel Cells
• Supercapacitors
• SMES
• Compressed air
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Distributed Generation
•Used to provide “clean power” to critical loads, isolating them from disturbances
with origin in the grid.
• Backup generators to assure energy supply to critical loads during sustained outages.
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Flywheels
Electromechanical device that couples a rotating electric machine
(motor/generator) with a rotating mass to store energy for short durations.
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Source: http://www.beaconpower.com
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Flywheels
Working Principle
Source: http://www.beaconpower.com
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Supercapacitors
New technology applied to capacitors
• High power density
• Long life and non-toxic
UPS with supercapacitors ESMA
(1 MJoule, 1000 kg)
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Superconducting Magnetic Energy
Storage (SMES)
Energy is stored in the magnetic field of a coil made of superconductor material.
• High power density
• Very fast response
• Very expensive (on development)
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Compressed Air
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Energy storage technologies comparison
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Constant voltage transformers (CVT)
Use the principles of resonance and core saturation to provide an output voltage
approximately constant, when the input voltage decreases.
Simple technology
Low price
Low efficiency (80% at full load)
Can produce harmonics and transients
Vibration and noise, when resonance
occurs
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Voltage compensator
Shunt voltage compensator
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Series voltage compensator
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Passive harmonic filters
Low price
Available for high equipment power
It’s necessary one circuit to filter each
harmonic
Ineffective if load conditions are variable
Increases the probability of resonance
occurrence
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Active harmonic filters
Filters several harmonics at the
same time
Filtering independent of grid
topology and load conditions
Doesn’t increase the risk of
resonance
Can be used to compensate
other PQ disturbances (flicker,
unbalance)
Price
Limited equipment power
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Hybrid harmonic filters
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End-use Devices Less Sensitive
• In most cases, making the end-use devices less sensitive to PQ
disturbances is more cost effective than buying equipment to mitigate these
problems.
• Some measures to increase equipment immunity:
– Add a capacitor with larger capacity to power supplies;
– Use cables with larger neutral conductors;
– Derate transformers;
– Use of oversized active front-ends.
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